After looking at the performance metrics in the online ADC simulator tool it is a great time to look at another online ADC tool called the Frequency Folding Tool. It is a useful simulator that helps to understand the effects of aliasing in an ADC. This tool can be found at this link ADI Data Converter Tools. It will take you to the page shown below.
This page has several different tools and tutorials available. We will take a look at some of the other tools in later blogs, but let’s take some time here to look specifically at the Frequency Folding Tool. Once landing on the ADI Data Converter Tools page, scroll down to the bottom of the page to find the Frequency Folding Tool.
As you can see there are other ADC related tools such as the SNR/THD/SINAD calculator, the Sigma-Delta ADC Tutorial, SPIController, and VisualAnalog. The Third Party Tools link takes the user to a page where more information is available for integrating ADC models from ADI into other software environments such as Agilent ADS, AWR Visual System Simulator, National Instruments’ Signal Express, and Mathworks. There is also a link to the DAC Harmonic Analyzer. Clicking on the link for the Frequency Folding Tool will take the user to the page below.
In the example shown the sampling frequency is set to 737.28 MHz and the input tone is set to 449.1 MHz with an amplitude of -15 dBFS. The second harmonic level is set to –85 dB and the third harmonic level is set to –89 dB. The input in this case is a single tone carrier and, as shown, the tone falls into the second Nyquist zone (368.64 MHz to 737.28 MHz). The tone aliases into the first Nyquist zone (0 Hz to 368.64 MHz) at 288.18 MHz. The second harmonic (898.2 MHz) and third harmonic (1347.3 MHz) also alias into the first Nyquist zone and appear at 160.92 MHz and 127.26 MHz respectively. If we expand the number of Nyquist zones, we can see the second and third harmonics in real frequency rather than in just their aliased tones in the first Nyquist zone.
In this example, I’ve selected four Nyquist zones. The tool supports up to ten Nyquist zones, but for this example viewing the first four Nyquist zones is sufficient to see the second and third harmonics for the input frequency and sampling frequency that is selected. Notice that the even Nyquist zones (second and fourth) are colored to make them distinguishable from the first and third Nyquist zones. While on this view, the user may move the mouse pointer over the fundamental tone until the four-way arrow appears.
Using this arrow, the user may move the fundamental tone and observe the effects it has on the alias of the fundamental tone as well as the second and third harmonics. This can give a ‘real-time’ view of how the frequency range of the input tone will affect where the fundamental tone and its second/third harmonics will alias into the first Nyquist zone. Last, let’s take a quick look at a wideband carrier. For visibility, I have selected a 40 MHz wide input signal bandwidth even though it is unlikely a signal would be this wide (in communications, the widest LTE bandwidth I am aware of is 20 MHz).
Once again, the input tone is set to 449.1 MHz and we can see that the harmonics are centered in the same locations. The interesting thing to note with these input conditions is that the second and third harmonics aliased signals overlap each other in the first Nyquist zone. This means the energy in the overlapped region will be higher due to the summing of the signals in this region. We will leave the complex math for how these will sum out for the sake of simplicity here for the blog, but at least the user know that the tool will show if aliased signals will overlap.
The Frequency Folding Tool is quite a neat tool to learn from. It can give some interesting insight into aliasing in an ADC with both single tone and wideband input carriers. I hope that you take some time to navigate to this useful tool and learn how signals can alias with an ADC. Stay tuned as we continue to look at the many design tools that ADI has to offer.